A new approach of CeO_2 and La_2O_3 effects on the three-way catalysts containing low precious metals

A series of three-way catalysts (TWCs),containing a small amount of precious metals (PMs,including Pt,Pd and Rh) and a large amount of promoters CeO2 and La2O3,were prepared with different precursor compounds and various doped manners.Crystal phases,dispersion of cerium and lanthanum,textural structure and thermal stability of the catalysts were investigated by XRD,XPS and pore parameters determination.The catalytic performance was studied by the measurements of CO,C3H6 and NO conversions on dependence of temperature at stoichimetric number point (S=1.00),and from S=0.75 to 1.30 at 280℃ or 340℃ for fresh or aged samples,respectively.The correlation between the catalytic performance and the characteristics of fresh and aged samples were discussed.The results show that the sample,in which CeO2 and La2O3 are doped with mixed oxide powders,possesses poor dispersion and less thermal stability,and the conversions of NO and C3H6 are apparently lower than those of the samples aged at 850℃ The main reason is     

Thermal decomposition features of calcium and rare-earth oxalates

Thermal decomposition of Ln₂(C₂O₄)₃ · 9H₂O concentrate (Ln = La, Ce, Pr, Nd) in the presence of CaC₂O₄ · H₂O was studied by X-ray diffraction, thermogravimetry, and chemical analysis. Annealing at temperatures above 374°C in the absence of calcium oxalate gives rise to the solid solution of CeO₂-based rare-earth oxides. Calcite CaCO₃ is formed in the presence of calcium oxalate at annealing temperatures above 442°C, which impedes the formation of lanthanide oxide solid solution and favors crystallization of oxides as individual La₂O₃, CeO₂, Pr₆O₁₁, and Nd₂O₃ phases. An increase in temperature above 736°C is accompanied by decomposition of calcium carbonate to give rise to an individual CaO phase and an individual phase of CeO₂-based lanthanide oxide solid solution.     

铈铬比例对CeO2-CrOx复合氧化物催化剂上1,2-二氯乙烷催化氧化性能的影响

采用共沉淀法制备了不同铈铬比例的CeO₂-CrO_x复合氧化物(Ce/Cr 摩尔比分别为9/1、4/1、2/1、1/1、1/2、1/4、1/8)以及单纯的CeO₂和Cr₂O₃, 并研究了各催化剂对1,2-二氯乙烷(DCE)的催化氧化性能. 结果表明,相较于单纯的CeO₂, 不同铈铬比例的复合氧化物催化剂对DCE的催化氧化活性有明显提高, 其中Ce/Cr 摩尔比为2/1的CeO₂-CrO_x复合氧化物上DCE的氧化活性最好, 且只有极微量的含氯等副产物产生; 随着Ce/Cr 摩尔比减小, 对HCl的选择性有下降的趋势. 通过X射线衍射(XRD)、N₂吸附/脱附(BET)、紫外拉曼(UV-Raman)光谱、H₂程序升温还原(H₂-TPR)、NH₃程序升温脱附(NH₃-TPD)等实验技术, 研究了铈铬比例对铈铬复合氧化物的物理化学性质的影响. 结果表明, 适当比例Ce-Cr的复合, 形成了结构较稳定的Ce-Cr-O固溶体, 提高了催化剂活性氧物种的流动性, 催化剂表面酸量及强弱酸比例, 从而有利于DCE的吸附活化, 进一步脱氯降解以及深度氧化.     

CO oxidation over Au/CeO<Subscript>2</Subscript>-ZrO<Subscript>2</Subscript> catalists: The effect of the support composition of the au-support interaction

The effect of the support composition on the Au-support interactions and its role in the creation of the activity of Au/CeO₂-ZrO₂ catalysts in CO oxidation has been studied. The CeO₂-ZrO₂ oxides and Au/CeO₂-ZrO₂ catalysts were synthesized, characterized by BET, XRD, HRTEM, AAS, TPR-H₂, and tested in CO oxidation. An approximate evaluation of the H₂ consumption for the surface reduction of the studied samples was estimated applying the model developed by Johnson and Mooi, which is based on the qualitative relationship between the amount of the capping oxygen and BET surface area. The sequence of the increasing percentage of O₂ atoms in the capping peak to the total Ce atoms follows the sequence of the decreasing Zr/Ce molar ratio in the sample. The activity of Au/CeO₂-ZrO₂ catalysts depends on the support composition and increases with the decrease in Zr/Ce molar ratio.     

ACID-base properties of a CERIA-lanthana catalytic system

A ceria-lanthana system was prepared by the sol-gel technique in order to obtain active catalysts for the 4-methylpentan-2-ol conversion. As the products distribution strongly depends on the acid-base features of the catalyst, acidity and basicity of the CeO₂-La₂O₃ samples were determined by adsorption microcalorimetry, using ammonia and carbon dioxide as probe molecules. Both concentration and strength of the sites were assessed and their nature was investigated by analysing the microcalorimetric data in the light of the structural and textural characterisation previously carried out. Present samples are compared to a formerly investigated ceria-lanthana system prepared by a different procedure. This revised version was published online in July 2006 with corrections to the Cover Date.     

EXAFS study of ceria-lanthana-based TWC promoters prepared by sol-gel routes

Extended X-ray absorption fine structure (EXAFS) experiments at the Ce K- and La K-edges were performed on ceria-lanthana-alumina three-way catalysts promoters prepared by sol-gel routes, in order to investigate the effect of lanthanum doping on the ceria structure. The formation of Ce_1−xLa_xO_2−x/2 solid solution, already observed by X-ray diffraction, was confirmed by EXAFS analysis, while no experimental evidence of a Ce-Al interaction was found. In presence of cerium and aluminum, lanthanum is involved in the formation of solid solution with CeO₂ and of La-Al compounds. When the La:Al molar ratio is sufficiently high, the growth of a tridimensionally ordered LaAlO₃ perovskite compound is observed. For increasing values of x/1−x in the solid solution Ce_1−xLa_xO_2−x/2, the Ce-O distance decreases, while La-O distance remains nearly constant.     

Different Synthesis Protocols for Co3O4–CeO2 Catalysts—Part 1: Influence on the Morphology on the Nanoscale

Co₃O₄‐modified CeO₂ (Co/Ce 1:4) was prepared by a combination of sol–gel processing and solvothermal treatment. The distribution of Co was controlled by means of the synthesis protocol to yield three different morphologies, namely, Co₃O₄ nanoparticles located on the surface of CeO₂ particles, coexistent Co₃O₄ and CeO₂ nanoparticles, or Co oxide structures homogeneously distributed within CeO₂. The effect of the different morphologies on the properties of Co₃O₄–CeO₂ was investigated with regard to the crystallite phase(s), particle size, surface area, and catalytic activity for CO oxidation. The material with Co₃O₄ nanoparticles finely dispersed on the surface of CeO₂ particles had the highest catalytic activity.     

Synthesis and Characterization of Fluorite-Like Ce-Zr-Y-La-O Systems

Ce-Zr-O and Ce-Zr-Y-La-O materials obtained under various conditions and at varying component ratios are characterized. At Ce/Zr ≈ 1, a tetragonal phase that can hardly be distinguished from a cubic phase by X-ray diffraction forms in the ternary system. Raising the precipitation temperature favors the formation of two-phase systems. Promoting the Ce/Zr = 0.26–0.62 materials with both yttrium and lanthanum favors the formation of a single-phase specimen, namely, a (Ce, Zr, Y, La)O₂ fluorite-like solid solution at 600°C. This structure persists up to 1150°C. The specific surface area of the (Ce, Zr, Y, La)O₂ materials is primarily determined by their calcination temperature: S_sp = 50–80 m²/g at 600°C and 0.6–0.8 m²/g at 1150° C. The specimens calcined at 600°C are mesoporous, with uniformly sized pores of mean diameter 32 ± 2 Å, and have no micropores. According to TPR data, the specimens calcined at 600°C are reduced with hydrogen in two steps that can apparently be interpreted as surface and bulk reduction. The Ce/Zr = 0.26 and 0.40 specimens calcined at 1150°C are reduced in a single step, giving rise to TPR peaks at 707 and 686°C, respectively, and their degree of reduction increases with decreasing Ce/Zr.     

Adsorption and migration growth of the Ce,O, and F adatoms on the CeO2 (111) surface: A density functional theory study

In order to investigate the microscopic behavior of the crystal surface growth of the fluorinated cerium dioxide polishing powder, the adsorption and migration of the Ce, O, and F atoms on the CeO₂ (111) surface were studied by using density functional theory with Hubbard correction +U. The adsorption energies of three single atoms at five high-symmetry sites and the migration activation energies along the migration pathway on the CeO₂ (111) surface were calculated. Results show that the most stable adsorption sites of the Ce, O, and F atoms were the O_h, Ce_bri, and Ce_t sites, respectively. The Ce atom migrated from the O_h to the O_t site. The O atom migrated from the Ce_bri to the O_bri site. The F atom migrated from the Ce_t to the O_h site. The migration activation energies of the Ce, O, and F atoms along the migration pathways were 1.526, 0.597, and 0.263 eV, respectively. The F adatom does not change the spatial configuration of the Ce and the O atoms. When the O vacancy occurs on the CeO₂ (111) surface, the F adatom can make up for the O vacancy defect.     

高热稳定性的铈铝复合氧化物用于柴油机尾气中碳烟的消除

采用柠檬酸络合燃烧法制备了一系列铝铈复合氧化物(铝掺杂的氧化铈),并通过程序升温氧化反应在紧密接触的模式下研究了其催化氧化碳烟的活性.结果表明,氧化铝和氧化铈之间存在强烈的相互作用,部分铝可以进入氧化铈晶格形成铝铈固溶体,大部分铝以γ-Αl2O3形式存在.与纯氧化铈相比,铝铈复合氧化物具有较好的催化燃烧活性,这是由于γ-Αl2O3能作为"扩散阻碍"阻止氧化铈粒子之间的接触而增强其热稳定.晶格氧的活动性决定了铝铈复合氧化物的催化活性,当铝与铈的摩尔比为1:30时,复合氧化物的催化活性最高.     

合成方法对Rh/CeO2-ZrO2-La2O3催化剂的结构、表面性能及DeNOx活性的影响

用沉积沉淀法合成两种不同系列的CeO2-ZrO2-La2O3混合氧化物(ZrO2和La2O3沉积CeO2粒子(标记为A-x)以及CeO2和La2O3沉积ZrO2粒子(标记为B-x)),并用作Rh催化剂的载体。XRD、拉曼、TPR、XPS和O2脉冲等表征结果显示出不同的沉积顺序将导致不同的结构和氧化还原性能,且B-x具有更高的氧迁移性、储氧能力和表面Ce浓度。当其负载Rh后,Rh/B-x催化剂具有更高的NO和CO转化率及N2选择性,且Ce的最佳含量为50at%。这可能归因于Rh负载于富铈表面形成更多有利于NO分解的表面Ce3+活性位。     

Physicochemical and catalytic properties of systems based on CeO<Subscript>2</Subscript>

The effect of synthesis conditions, the nature of components, and the ratio between the components on the phase composition, the texture, and the redox and catalytic properties of the Ce-Zr-O, Ce-Zr-M₁-O (M₁ = Mn, Ni, Cu, Y, La, Pr, or Nd), N/Ce-Zr-O (N = Rh, Pd, or Pt), and Pd/Ce-Zr-M₂-O/Al₂O₃ (M₂ = Mg, Ca, Sr, Ba, Y, La, Pr, Nd, or Sm) was considered. A cubic solid solution with the fluorite structure was formed on the introduction of <50 mol % zirconium into CeO₂, and the stability of this solid solution depended on preparation procedure and treatment conditions. The presence of transition or rare earth elements in certain concentrations extended the range of compositions with the retained fluorite structure. The texture of the Ce-Zr-O system mainly depended on treatment temperature. An increase in this temperature resulted in a decrease in the specific surface area of the samples. The total pore volume varied over the range of 0.2–0.3 cm³/g and depended on the Ce/Zr ratio. The presence of transition or rare earth elements either increased the specific surface area of the system or made it more stable to thermal treatment. The introduction of the isovalent cation Zr⁴⁺ into CeO₂ increased the number of lattice defects both on the surface and in the bulk to increase the mobility of oxygen and facilitate its diffusion in the Ce_{1 − x} Zr _x O₂ lattice. The catalytic properties of the Ce-Zr-M₁-O or N/Ce-Zr-M₂-O systems were due to the presence of anion vacancies and the easy transitions Ce⁴⁺ ai Ce³⁺, M₁²ⁿ⁺ ai M₁ ⁿ⁺, and N ^δ+ → N ⁰ in the case of noble metals.     

Effect of promoters on the catalytic performance of Ni/Al2O3 catalyst for partial oxidation of methane to syngas

The effect of promoters such as Ce, La and Ca on catalytic performance of Ni catalyst was measured in a continuous fixed bed reactor. The effect of promoters on Ni/a-Al₂O₃ catalyst is more significant than on Ni/g-Al₂O₃ catalyst. Ce was proved to be the best promoter among the three promoters tested and the optimum loading of Ce was 1%. The catalyst was characterized by TG, XPS, TPR and XRD techniques. TPR results showed that Ce can improve the reducibility of the Ni/Al₂O₃ catalyst. XRD results indicated that Ce was highly dispersed when its loading was low, but at higher loading it was crystallized into bulk CeO₂, thus, decreased the catalytic activity. This revised version was published online in June 2006 with corrections to the Cover Date.     

不同载体Ni基负载型催化剂对甲烷部分氧化制合成气催化行为研究

采用浸渍法制备了单一载体(Al₂O₃、ZrO₂、CeO₂)和ZrO₂、CeO₂改性的Al₂O₃复合载体的Ni催化剂,考察了在甲烷部分氧化制备合成气反应中的催化性能。通过N₂-物理吸附、H₂程序升温还原、X射线衍射、NH₃程序升温脱附和程序升温氧化等技术对催化剂进行了表征。结果表明,在单一载体催化剂中,Ni/Al₂O₃具有较大的比表面积,其初始反应活性较高,但该催化剂表面易形成大量的积炭而快速失活。Ni/ZrO₂和Ni/CeO₂催化剂比表面积较小,活性金属Ni在其表面分散性差,催化剂具有较低的CH₄转化率。而CeO₂和ZrO₂改性的Al₂O₃复合载体催化剂,具有较大的比表面积,反应活性明显高于单一载体催化剂。CeO₂-Al₂O₃复合载体催化剂具有最高的反应活性和较好的反应稳定性。同时表明,含CeO₂催化剂反应后表面积炭较少,CeO₂的储放氧功能增强了催化剂对O₂的活化,提高催化剂活性的同时,可以抑制积炭的生成。     

Thermal decomposition of Y,La and lanthanide benzene-1,2-dioxyacetates in an air atmosphere

The thermal decompositions of Y, La and lanthanide (from Ce(III) to Lu(III) benzene-1,2-dioxyacetates with general formula Ln₂(C₁₀H₈O₆)₃·nH₂O were studied. The hydrated complexes first lose water of crystallization in one or two steps to yield anhydrous compounds or hydrates containing coordination water molecules, and then decompose to the oxides Ln₂O₃, CeO₂, Pr₆O₁₁ and Tb₄O₇ with formation of intermediates, carbonates and oxycarbonates (La, Pr-Eu), oxycarbonates (Y, Tb-Lu) or carbonate (Gd) only. Anhydrous cerium(III) benzene-1,2-dioxyacetate decomposes on heating directly to CeO₂.     

Surface structure and redox chemistry of ceria-containing automotive catalytic systems

A summary is given of recent work done at the authors' laboratory, using several spectroscopies, molecular modeling and catalytic activity tests, to study the surface reactivity of ceriacontaining high surface area solids useful for automotive exhaust depollution catalysis. Issues addressed include the influence of the basicity of the cerium ion ligand environment on its ability for activation of O₂ at the material surface, and its modification by supporting on alumina or doping with Cl⁻ or other anions; the tendency to grouping of oxygen vacancies at the CeO₂ (111) surface; the higher ease for formation of surface reduced centers in mixed (Zr,Ce) oxides; and the effect of electronic transfer and other chemical interactions induced by ceria on the redox reactivity of transition metalcontaining catalysts.     

Synthesis of ceria–zirconia mixed oxide from cerium and zirconium glycolates via sol–gel process and its reduction property

Ceria–zirconia mixed oxide was successfully synthesized via the sol–gel process at ambient temperature, followed by calcination at 500, 700 and 900 °C. The synthesis parameters, such as alkoxide concentration, aging time and heating temperature, were studied to obtain the most uniform and remarkably high‐surface‐area cubic‐phase mixed oxides. The thermal stability of both oxides was enhanced by mutual substitution. Surface areas of the Ce_xZr_1?xO₂ powders were improved by increasing ceria content, and their thermal stability was increased by the incorporation of ZrO₂. The most stable cubic‐phase solid solutions were obtained in the Ce range above 50 mol%. The highest surface area was obtained from the mixed catalyst containing a ceria content of 90 mol% (200 m²/g). Temperature programmed reduction results show that increasing the amount of Zr in the mixed oxides results in a decrease in the reduction temperature, and that the splitting of the support reduction process into two peaks depends on CeO₂ content. The CO oxidation activity of samples was found to be related to its composition. The activity of catalysts for this reaction decreased with a decrease in Zr amount in cubic phase catalysts. Ce₆Zr₄O₂ exhibited the highest activity for CO oxidation. Copyright © 2006 John Wiley & Sons, Ltd.     

Thermal decompositions of light lanthanide aconitates

The conditions of thermal decomposition of Y, La, Ce(III), Pr, Nd, Sm, and Gd aconitates have been studied. On heating, the aconitate of Ce(III) loses crystallization water to yield anhydrous salt, which then is transformed in to oxide CeO₂. The aconitates of Y, Pr, Nd, Sm, Eu and Gd decompose in three stages. First, aconitates undergo dehydration to form the anhydrous salts, which next decompose to Ln₂O₂CO₃. In the last one the thermal decomposition of Ln₂O₂CO₃ to Ln₂O₃ is accompanied by endothermic effect. Dehydration of aconitate of La undergoes in two stages. The anhydrous complex decomposes to La₂O₂CO₃; this subsequently decomposes to La₂O₃.     

过渡金属氧化物修饰石墨毡阴极及电催化氧化性能测试

采用H2O2化学预处理石墨毡,并将过渡金属氧化物Ce O2负载到石墨毡上,制备出复合石墨毡阴极材料。研究结果表明H2O2处理可增加石墨毡的含氧官能团,改善表面亲水性,进而提高Ce O2的负载量,XRD分析表明石墨毡表面负载的Ce O2为萤石结构。电化学阻抗谱(EIS)和循环伏安曲线(CV)分析表明修饰后的石墨毡电荷传输阻力变小,氧化还原电流强度显著增强,活性表面积增大8倍,线性扫描(LSV)实验表明改性石墨毡在氧还原过程中具有较大的电流密度,是未改性前的8.5倍。采用改性石墨毡作为阴极,进行电芬顿催化降解甲基橙测试,20 min脱色率达到96.8%,与未改性石墨毡相比,去除率提高133.2%,显著提高了其电催化氧化性能。     

过渡金属氧化物修饰石墨毡阴极及电催化氧化性能测试

采用H₂O₂化学预处理石墨毡,并将过渡金属氧化物CeO₂负载到石墨毡上,制备出复合石墨毡阴极材料。研究结果表明H₂O₂处理可增加石墨毡的含氧官能团,改善表面亲水性,进而提高CeO₂的负载量,XRD分析表明石墨毡表面负载的CeO₂为萤石结构。电化学阻抗谱(EIS)和循环伏安曲线(CV)分析表明修饰后的石墨毡电荷传输阻力变小,氧化还原电流强度显著增强,活性表面积增大8倍,线性扫描(LSV)实验表明改性石墨毡在氧还原过程中具有较大的电流密度,是未改性前的8.5倍。采用改性石墨毡作为阴极,进行电芬顿催化降解甲基橙测试,20 min脱色率达到96.8%,与未改性石墨毡相比,去除率提高133.2%,显著提高了其电催化氧化性能。